A Novel Ambipolar Nonaqueous All-Organic Redox Flow Battery and Its State of Charge Detection By FTIR

Nonaqueous redox flow batteries (NARFB) are considered as a promising next-generation grid scale energy storage solution because of their potential higher energy densities derived from the wide voltage window of nonaqueous electrolytes. Current research of NARFB is focused on redox materials development for various flow battery chemistries. Flow battery operation involves complexity associated with solvent-salt-redox species interactions, porous electrode, flow field, material crossover, etc., making it challenging to diagnose the real-time battery status. However, there is critical knowledge and technical gaps in this aspect. Especially, fast, robust and cost-effective diagnostic techniques are lacking to monitor state of charge (SOC) in a real-time manner to maintain safe and reliable long-term flow battery operations.

In this research, we report a novel all-organic redox flow battery based on an ambipolar redox material and demonstrate Fourier Transform infrared spectroscopy (FTIR) as an effective method of detecting the SOC of this flow battery.  The concentrations of the redox materials were derived from FTIR signal intensity by using the Beer-Lampert Law, which were used to calculate the SOC of the flow battery during the charge/discharge processes. The FTIR-based SOC determination agrees well with the electron spin resonance (ESR)-based measurements, demonstrating the feasibility of FTIR for online monitoring of the flow battery operation status. Also, the evolution of the FTIR spectra over repeated flow battery cycling offers a possible way to study the redox materials degradation and battery performance fading mechanism. This state of health (SOH) study can provide a direct prediction of cycle life of a NARFB.